Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images which show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions which emanate from the body and are captured by a scintillation crystal, with which the photons interact to produce flashes of light or “events.” Events are detected by an array of photodetectors, such as photomultiplier tubes, and their spatial locations or positions are calculated and stored. In this way, an image of the organ or tissue under study is created from detection of the distribution of the radioisotopes in the body.
One particular nuclear medicine imaging technique is known as Positron Emission Tomography, or PET. PET is used to produce images for diagnosing the biochemistry or physiology of a specific organ, tumor or other metabolically active site. Measurement of the tissue concentration of a positron emitting radionuclide is based on coincidence detection of the two gamma photons arising from positron annihilation. When a positron is annihilated by an electron, two 511 keV gamma photons are simultaneously produced and travel in approximately opposite directions. Gamma photons produced by an annihilation event can be detected by a pair of oppositely disposed radiation detectors capable of producing a signal in response to the interaction of the gamma photons with a scintillation crystal. Annihilation events are typically identified by a time coincidence between the detection of the two 511 keV gamma photons in the two oppositely disposed detectors, i.e., the gamma photon emissions are detected virtually simultaneously by each detector. When two oppositely disposed gamma photons each strike an oppositely disposed detector to produce a time coincidence event, they also identify a line of response, or LOR, along which the annihilation event has occurred.
An example of a PET method and apparatus is described in U.S. Pat. No. 6,858,847, which patent is incorporated herein by reference in its entirety. After being sorted into parallel projections, the LORs defined by the coincidence events are used to reconstruct a three-dimensional distribution of the positron-emitting radionuclide within the patient. PET is particularly useful in obtaining images that reveal bioprocesses, e.g. the functioning of bodily organs such as the heart, brain, lungs, etc. and bodily tissues and structures such as the circulatory system.
On the other hand, Magnetic Resonance Imaging (MRI) is primarily used for obtaining high quality, high resolution anatomical and structural images of the body. MRI is based on the absorption and emission of energy in the radio frequency range primarily by the hydrogen nuclei of the atoms of the body and the spatial variations in the phase and frequency of the radio frequency energy being absorbed and emitted by the imaged object. The major components of an MRI imager include a cylindrical magnet, gradient coils within the magnet, an RF coil within the gradient coil, and an RF shield that prevents the high power RF pulses from radiating outside of the MR imager, and keeps extraneous RF signals from being detected by the imager. A patient is placed on a patient bed or table within the magnet and is surrounded by the gradient and RF coils.
The magnet produces a Bo magnetic field for the imaging procedure. The gradient coils produce a gradient in the Bo field in the X, Y, and Z directions. The RF coil produces a B1 magnetic field necessary to rotate the spins of the nuclei by 90° or 180°. The RF coil also detects the nuclear magnetic resonance signal from the spins within the body. A radio frequency source produces a sine wave of the desired frequency.
The concept of merging PET and MR imaging modalities into a single device is generally known in the art. See, e.g., U.S. Pat. No. 4,939,464, incorporated herein by reference in its entirety. The original ideas focused around transporting the light from scintillation crystals positioned inside the MR field of view, via optical guides such as lightpipes, to photomultiplier tube photodetectors positioned outside of the magnetic field. This was necessary because the photomultiplier tubes are sensitive to magnetic field disturbance. However, such designs have not been able to achieve high performance characteristics and suffer from complicated design requirements.